The present invention relates to a photopolymerizable recording material comprising a layer support, a photopolymerizable layer and a cover layer possessing low permeability to atmospheric oxygen. The material is especially suited for the production of printing plates, in particular planographic printing plates.
Printing plates of the type mentioned are known, for example from U.S. Pat. No. 3,458,311. The cover layers described therein consist of water-soluble polymers, for example polyvinyl alcohol, polyvinylpyrrolidone, gelatin or gum arabic. Of these substances, polyvinyl alcohol is preferred, since its barrier effect in respect of oxygen is very high.
Compared to negative working planographic printing plates based on diazonium compounds, most of the presently produced planographic printing plates based on photopolymerizable materials yield a clearly reduced image resolution. It is assumed that this loss in resolution is partly due to a dark reaction propagating in the layer after the light-induced polymerization of the image areas. Scattered light reflected by the support surface is also supposed to be one of the causes. Thus, the resolution can be improved by providing anti-halation layers between the support surface and the photopolymerizable layer or by adding dyes or UV-absorbers to the photopolymerizable layer itself. For this purpose, dyes or pigments absorbing in the wavelength range to which the layer is sensitive are generally used. Materials of this type are, for example, described in U.S. Pat. No. 2,791,504, U.S. Pat. No. 4,173,673, DE-A 22 02 360 and DE-A 26 58 422. The dyes described therein in particular reduce the proportion of scattered light caused by reflection at the support surface. Improved resolution as a result of the addition of dyes to the photosensitive layer itself is in particular possible using dyes which absorb in the range of 350 to 450 nm. However, the improved resolution always results in a considerably reduced photosensitivity.
U.S. Pat. No. 3,218,167 discloses a photopolymerizable material for the production of transfer images by means of the peel-apart process. Since this process requires a pronounced image contrast, the photopolymerizable layer must be strongly dyed by the addition of dyes. In order to avoid a severe reduction of the photosensitivity, the dye must absorb as little as possible in the actinic range of the photopolymer layer. But as most of the dyes employed still absorb relatively much light in the activating range of the photoinitiator, the photosensitivity is significantly reduced in general. No other effect of the dye additives is mentioned.
It is also known to inhibit the dark reaction by adding polymerization inhibitors and thus to improve the resolution. When these inhibitors are highly effective, they have in most cases the disadvantage of reducing the photosensitivity. Moreover, many of these inhibitors result in an undesirable discoloring of the layer after a certain storage period. A great number of the known inhibitors absorb in the wavelength range of 300 to 700 nm, which is important for the initiation of the photopolymerization, and thus disturb or inhibit the imagewise polymerization.
From International Patent Application WO 86/7473 it is known to provide positive-working photoresist materials based on naphthoquinonediazides with a contrast-enhancing cover layer which is bleached by exposure to light in the actinic region. The resolution of these highly resolving materials is further improved by the increased image contrast.
WO 86/6182 describes a color proofing method where an optical filter layer is provided between the positive working photosensitive layer of the color proofing film and the color separation, in order to prevent halftone dot gain and to extend the exposure latitude. The filter layer must absorb light in the region of maximum absorbance of the photosensitive layer.
DE-C 21 23 702 describes a photopolymerizable dry resist material where a release layer containing a compound of low permeability to oxygen is present between the peelable temporary support film and the photopolymerizable layer. The release layer may be dyed with small amounts of a dye which is, however, not claimed to have any particular effect.